That answer is a bit dicier because it can change, depending on how you look at it. If you count TV appearances, his daughter Patricia Hitchcock takes the cake, with three movie credits and 10 roles in Alfred Hitchcock Presents. But let's stick to the movies.
My answer is Clare Greet, who had speaking roles in a total of eight Hitchcock films (Number 13 (1922), The Ring (1927), The Manxman (1929), Murder! (1930), The Man Who Knew Too Much (1934), Sabotage (1936) and Jamaica Inn (1939), plus Lord Camber's Ladies (produced by Hitch, 1932).
Clare Greet helped Hitch out and he never forgot it.
In Number Thirteen, her first film with Hitchcock, Greet had been awarded the starring role. His uncle, John Hitchcock, had invested in the movie and when that money ran out, Greet kicked in more funds. Unfortunately, it wasn't enough. Filming was brought to a halt and both of those individuals lost their investment. Humiliated, Hitch took that lesson to heart and he tried never to lose an investor's money again, earning a reputation for shooting his films on or under schedule and budget and aiming for commercial success – which he usually got. Hitch never forgot Greet's generosity and the belief in his talents that it implied and he repaid the favor by offering her more acting roles in his films than anyone else.
As an extra, Bess Flowers usually played what Sinatra would have a called a “classy broad.”
That said, there's another woman who could also contend for that top spot. “Queen of the Hollywood Extras,” Bess Flowers, appeared in over 700 movies in her career as an uncredited, usually non-speaking, walk-on. She can be spotted in seven Hitchcock films: (Mr. and Mrs. Smith (1941) as nightclub extra, Notorious (1946) as a party guest, Dial M for Murder (1954) as a woman departing the ship, Rear Window (1954) as a guest with a poodle at the songwriter's party, To Catch a Thief (1955) as a guest at the costume ball, The Man Who Knew Too Much (1956) as an attendee at the Royal Albert Hall concert, and Vertigo (1958) as a diner at Ernie's) and one episode of Alfred Hitchcock Presents (“The Legacy” (1956) as a diner at the hotel restaurant).
Finally, we shouldn't forget (though at this point it could be an uphill battle) Hannah Jones, who appeared in six of Hitchcock's early films: Downhill (1927), Champagne (1928), Blackmail (1929), Elstree Calling (1930), Murder! (1930) and Rich and Strange (1931). She missed the number one spot by one film, but in case the name comes up on Jeopardy, there you go.
I'd love to hear about your interview with Pat... can you send it along? I'll post it.